1. Field of the Invention
The present invention relates to a package to be used for telecommunication or the like, and in particular, a package to be mounted with a semiconductor which requires heat dissipation performance, and a method of manufacturing the package.
2. Description of the Related Art
Conventionally, for a package to be mounted with a semiconductor, a method has been used whereby a plate of CuW and a plate of AlN ceramics, which have excellent heat dissipation performance, are used for the bottom of the package so as to dissipate heat generated due to the operation of the semiconductor. Such a bottom plate is called a substrate, and heat dissipation performance has been improved by employing these substrates in comparison with substrates of alumina-ceramics, an Fexe2x80x94Nixe2x80x94Co alloy (trade name: Covar), and a 42 alloy (Fexe2x80x94Ni alloy).
Furthermore, a thermo electric cooler using the Peltier effect has been used. This cooler uses a phenomenon called the xe2x80x9cPeltier effectxe2x80x9d. When a current is supplied to a semiconductor, the heat of one side semiconductor is carried to another side. The heat absorbing side of the semiconductor is cooling. The thermo electric cooler combined with this semiconductor has a significant effect, however, in that its thickness increases since the device itself requires a combination of the semiconductor device and a substrate.
However, since entering year 2000, multiplex telecommunication in which the signal channel is great has become the mainstream in the field of telecommunication systems. For example, the wavelength division multiplexing (WDM) method is rapidly spreading in the field of optical communications, and the wide Band-CDMA method which has been used in cellular phones is rapidly growing in the field of radio communications.
In these markets, in order for the device to satisfy performance of high output and low-distortion, it is necessary to lower the operating temperature of the device. As a means for accomplishing this, the heat dissipation performance of the package has been improved, and it is particularly important that the heat dissipation from a heatsink or the like, on which a thermo electric cooler is mounted, is improved heat absorbing ability toward the direction of the substrate. AlN ceramics and CuW which are the materials of the abovementioned substrates also have a sufficient effect for improving the heat dissipation performance, however, a device with a high output whose heat dissipation performance is further improved has been demanded.
As one of means for satisfying this demand, electric cooling using the above-mentioned thermo electric cooler is used. However, since the device includes a combination of a semiconductor and a substrate, the volume increases and electric circuits for cooling become necessary in comparison with the case of using only one substrate.
In addition, in Japanese Unexamined Patent Publication No. 345922 of 1999, a package using diamond which has excellent heat dissipation performance is disclosed. In this example, vapor-phase synthetic diamond and a metal member with high heat conductivity are laminated to improve heat dissipation performance, however, since a diamond having a considerably larger area than that of a semiconductor device to be mounted is used, although the cost of this package can be reduced, the reduction in the cost is limited.
In order to use a material as a substrate of a package, it is necessary to improve the heat conductivity of the material to be used as a substrate. As materials with high heat conductivity, diamond can be used as well as metals including Au, Ag, Cu and others. However, a diamond plate or a diamond-coated plate is costly. Consequently, the use of diamond as a material composing a large substrate is too high-cost.
The invention achieves a means for improving heat dissipation performance while reducing the amount of use of an expensive diamond plate or diamond coated plate. In the means, a diamond plate or diamond coated plate is used as a substrate immediately under a highly heated portion of an IC. Concretely, the substrate is formed by connecting two or more kinds of substrates which are different in heat conductivity.
Concretely, a substrate of one kind selected from plates of diamond coated AlN ceramics, diamond coated Si, and diamond and one or more kinds of substrates selected from AlN ceramics, Si, and CuW are combined and used. The diamond coated portion or the diamond plate portion is positioned immediately under the intensest heated portion of the IC. That is, in the package, the substrate with high performance heat dissipation is disposed under a portion of which the temperature may become especially high. For a substrate to be disposed at a position other than the position immediately under the highly heated portion, one or more kinds of substrates of AlN ceramics, Si, and CuW are used. In a case where the diamond plate is used, any kind of substrate can be used, however, in a case where the diamond coated substrate is used, it is preferable that the same material as the coated material is used. Needless to say, the diamond substrate and the diamond coated substrate may be combined and used.
In all cases, the method of manufacturing the package is as follows. First, a ceramics frame which has been subjected to an electrode frame of the package is combined with external leads and fixed. At the time of attaching substrates on the fixed ceramics frame, two or more kinds of substrates are prepared, and lined-up and adhered on the substrate""s mountable side of the frame. At the same time, the substrates are connected to each other by brazing. After adhering, in order to make the thicknesses of the substrates even, the backside surfaces of the package frame are ground to be flat. At this time, in order to secure heat dissipation performance which is the object of the invention, it is preferable that a diamond plate or diamond coated substrate is used for a part of the substrate. In addition, the coated diamond is disposed so as to face the interior of the package. Two or more kinds of substrates which are different in heat conductivity are used as the substrate, and in order to suppress distortion due to a difference in the thermal expansion coefficient from diamond, it is preferable that substrates of AlN ceramics, Si, and CuW are selected and used which have a small difference in the thermal expansion coefficient from diamond.
These materials have excellent heat conductivity and a small thermal expansion coefficient, and in comparison with the diamond""s thermal expansion coefficient of 2.3xc3x9710xe2x88x926/xc2x0 C., the thermal expansion coefficient of AlN is 4.5xc3x9710xe2x88x926/xc2x0 C., the thermal expansion coefficient of Si is 3.3xc3x9710xe2x88x926/xc2x0 C., and the thermal expansion coefficient of CuW is approximately 6.5xc3x9710xe2x88x926/xc2x0 C. in the case where the Cu ratio in the composition is small although it changes depending on its composition. Therefore, by using these materials, distortion due to the difference in the thermal expansion coefficient can be prevented provided that the difference in the temperature between the materials and diamond does not increase.
The package of the invention has a heat dissipation performance superior to that of the conventional package, and its distortion due to heat generation is within an allowable range, and the cost of the package can be reduced by minimizing the amount of expensive diamond to be used.